264 PATTERNS AND PROBLEMS OF DEVELOPMENT 



for the conclusion that the local differences in susceptibility to lithium or 

 other agents constitute evidence for the presence of local specific differ- 

 ences in the embryo. Considering one case, for example, Lehmann finds 

 that treatment with lithium at the beginning of gastrulation produces 

 otocephaly with anterior head region normal, while lithium treatment at 

 the midgastrula stage produces cyclopia, with otic region normal. The 

 possibility that in the first case early invagination of the part of the dorsal 

 inductor which attains later the most anterior position may protect it 

 to some extent from lithium action does not seem to be considered. If in- 

 vagination does protect in this way, Lehmann 's results may be a matter 

 of differential exposure to lithium rather than of specific regional differ- 

 ence. The possibility that different degrees of recovery may occur in dif- 

 ferent regions is also not considered. In the second case, lithium treat- 

 ment at midgastrula stage, anterior head region, and otic region may be 

 more or less equally exposed, and the more susceptible anterior region 

 more inhibited. However, cyclopia may also result from exposure to in- 

 hibiting agents beginning in early cleavage stages. 



Formation of notochord may also be inhibited by lithium treatment, 

 according to Lehmann, the prospective notochord being "mesodermized" 

 and incorporated into the somites, and these become continuous across 

 the median plane. Continuity of somites across the median plane has 

 been obtained by lithium treatment of late blastulae or early gastrulae of 

 Ambly stoma with retarded development, not absence, of notochord 

 (Cohen, 1938a). In these individuals the notochord has evidently not 

 been mesodermized, and transverse continuity of somites dorsal to it is 

 apparently a matter of physical conditions resulting from retardation of 

 its development. It is suggested by Cohen that in Lehmann 's material 

 the notochord is not mesodermized but is never segregated from the 

 archenteric roof, that is, under the inhibiting conditions it remains ento- 

 derm. The modifications of anuran development, evidently differential 

 inhibitions, obtained by Hoadley (1938) with high temperature, show 

 similar inhibitions of notochord and median continuity of somites. Evi- 

 dently this modification is not specific for lithium, and the fact that it is 

 more conspicuous anteriorly suggests differential susceptibility in relation 

 to an anteroposterior gradient in the invaginated chordamesoderm ; the 

 presence of such a gradient is shown by differential lethal susceptibility 

 and by differential dye reduction and is also suggested by the different 

 degrees of inductor action by different portions of the dorsal inductor re- 

 gion when implanted. Once more it may be emphasized that what may 



